feat: make ephemeral env use supersetbot + deprecate build_docker.py

[mistercrunch]

Someone reported that ephemeral env builds are failing. Example action run here https://github.com/apache/superset/actions/runs/11721810688/job/32650049015#step:7:1219

The issue above seems to be a glitch/one-time-occurence, yet this PR is still relevant.

In this PR:

• allow the ephemeral-env workflow to be testable using workflow_dispatch
• aligns docker build to use supersetbot like the main docker-related workflows
• removes now obsolete python script build_docker, replaced by supersetbot docker now, along with references in the docs

test command ->

gh workflow run ephemeral-env.yml --ref fix_ephemerals --field comment_body="/testenv up" --field issue_number=30870

[mistercrunch]

/testenv up

[github-actions]

@mistercrunch Ephemeral environment spinning up at http://34.220.122.153:8080. Credentials are admin/admin. Please allow several minutes for bootstrapping and startup.

[mistercrunch]

Just a note that the test above operated from the main branch, since the changes are on the GHA workflow itself.

[mistercrunch]

BTW wasn't able to reproduce the issue, but this PR makes it easier to debug and aligns with using supersetbot to generate the docker command

[mistercrunch]

Latest test run -> https://github.com/apache/superset/actions/runs/11964471179/job/33357016642

[github-actions]

@ Ephemeral environment spinning up at http://54.202.175.138:8080. Credentials are admin/admin. Please allow several minutes for bootstrapping and startup.

[mistercrunch]

worked! Looks like there's some unrelated CI errors also seen on master